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A Key to Civil Architecture ; Or,

When his mind is at rest with respect to the basisof his building, he must then turn his thoughts tothe centring for the vaults. There are variousforms of those ; but the strongest, in my opinion, isthe circular; for if bricks or stones to those were cutwedge-wife, and disposed in the form of an arch allfrom one center, the bricks or stones can in no-wife 'sink downwards, for want of room to descend per-pendicular ; because all solid materials must descenddirectly downwards, for ponderosity hath a natu-ral tendency to the center of the world, and naturewill perform her works by the shortest lines ; neither ican the hutments of a semicircular arch suffer somuch as one made flatter, because the roundnessof it will rather incline the weight to rest upon, thanshove them out.

There is no difficulty in either making or settingof centers, but what every man that hath served a (time to the business must naturally know; notwith-standing, I shall not omit to acquaint my reader,that the most familiar method of setting centers, is,to cut pieces of quarter equal to the length at thetop and bottom, and set your bearers upon wedges,for the convenience of easily striking them; alsoif the vault is groined, to keep them up considera-bly in the middle, to prevent a defect in appearanceby the setting of the arch ; a thing very common 1in arches or vaults of a great span.

As some young men may be unacquainted withthe nature of striking out centers, the following isa certain method, for every part or a portion of acircle whatsoever.

EXAMPLE.